Water microjet photoemission experiments and scattering calculations reveal genuine properties of the hydrated electron.

Urea is a key molecule in the search for the origin of life and a basic chemical produced in large quantities by industry. Its formation from ammonia and carbon dioxide requires either high pressures and temperatures or, under milder conditions, catalysts or additional reagents. In this study, we observed the spontaneous formation of urea under ambient conditions from ammonia and carbon dioxide in the surface layer of aqueous droplets. Single, optically trapped droplets were probed by using Raman bands as markers. We found the surface layer to act like a microscopic flow reactor, with chemical gradients providing access to unconventional reaction pathways. This observation revealed a general mechanistic scheme for distinctive droplet chemistry. Interfacial chemistry is a possible nonenergetic route for urea formation under prebiotic conditions.

Optical confinement (OC) structures the optical field and amplifies light intensity inside atmospheric aerosol particles, with major consequences for sunlight-driven aerosol chemistry. Although theorized, the OC-induced spatial structuring has so far defied experimental observation. Here, x-ray spectromicroscopic imaging complemented by modeling provides direct evidence for OC-induced patterning inside photoactive particles. Single iron(III)–citrate particles were probed using the iron oxidation state as a photochemical marker. Based on these results, we predict an overall acceleration of photochemical reactions by a factor of two to three for most classes of atmospheric aerosol particles. Rotation of free aerosol particles and intraparticle molecular transport generally accelerate the photochemistry. Given the prevalence of OC effects, their influence on aerosol particle photochemistry should be considered by atmospheric models.

Photochemistry taking place in atmospheric aerosol droplets has a significant impact on the Earth’s climate. Nanofocusing of electromagnetic radiation inside aerosols plays a crucial role in their absorption behaviour, since the radiation flux inside the droplet strongly affects the activation rate of photochemically active species. However, size-dependent nanofocusing effects in the photokinetics of small aerosols have escaped direct observation due to the inability to measure absorption signatures from single droplets. Here we show that photoacoustic measurements on optically trapped single nanodroplets provide a direct, broadly applicable method to measure absorption with attolitre sensitivity. We demonstrate for a model aerosol that the photolysis is accelerated by an order of magnitude in the sub-micron to micron size range, compared with larger droplets. The versatility of our technique promises broad applicability to absorption studies of aerosol particles, such as atmospheric aerosols where quantitative photokinetic data are critical for climate predictions.